A government scientist kept silent about a potentially dangerous lab blunder and revealed it only after workers in another lab noticed something fishy, according to an internal investigation.

The accident happened in January at the headquarters of the Centers for Disease Control and Prevention in Atlanta. A lab scientist accidently mixed a deadly strain of bird flu with a tamer strain, and sent the mix to another CDC lab and to an outside lab in Athens, Ga.

No one was sickened by bird flu. But unsuspecting scientists worked with the viral mix for months before it was discovered.

CDC officials have called the incident the most worrisome in a series of lab safety problems at the government agency Earlier this summer, a lab mishandled anthrax samples and both the bird flu and anthrax labs were shut down.

"We all feel horrible this happened," said Dr. Anne Schuchat, who oversees the CDC's Influenza Division.

Because of employee privacy rules, she said she could not name the lab scientist or the leader of the scientist's team, who were both faulted in the investigation report released Friday. She said disciplinary actions are taking place, but she did not provide any details.

According to the report, the lab scientist was doing work with both bird flu strains - the deadly form and a tamer version. The lab scientist told investigators that the work was done in the proper sequence, but noted being rushed to finish the job and attend a meeting.

In February, some of the mixed virus sample was sent to another CDC lab in Atlanta. In March, a shipment of it went to a U.S. Department of Agriculture laboratory in Athens. For a study, the bird flu virus was given to chickens. The chickens died, prompting USDA staff to take a hard look at the sample and detect the deadly strain.

The USDA lab notified the CDC lab in May, and the CDC lab confirmed the finding.

But the CDC team leader didn't report what happened to supervisors or anyone else, reasoning that the viral mix was at all times contained in specialized laboratories and was never a threat to the public, the investigation report said.